A subwavelength structure is an optical structure having features smaller than a wavelength of an illuminating beam. Subwavelength structures may have periodically repeating features, either over the entire structure or with different periods over different portions of the structure. Subwavelength structures also may have more than one period spatially superimposed. The period of a uniformly periodic diffraction grating may be defined as the smallest distance over which the grating structure repeats. If the grating period of the subwavelength structure is less than half of the incident beam's wavelength, then only the zeroth diffraction order propagates, and all other diffraction orders are evanescent. Subwavelength structures show interesting properties such as antireflection, form birefringence, and emulation of distributed index materials.
Subwavelength structures have been used in the prior art, for example as antireflection surfaces. An example of this prior art technology is U.S. Pat. No. 5,007,708 to Gaylord et al., which describes a technique for producing antireflection grating surfaces on dielectrics, semiconductors and metals. However, these structures have been limited to rectangular geometries or stacks of rectangles. The resulting contour is, at best, a piecewise approximation of a smooth, analog, profile. In contrast, lithographic technology can fabricate smooth, analog profiles to achieve maximum transmission. The smooth, analog profiles attain maximum transmission over a wide range of structure depths. This feature makes these smooth analog profiles better suited to operate over large areas and high production volumes.
Lenses, microlenses, windows, sensing planes, and other products that are illuminated with light are often anti-reflection coated with a multi-layer dielectric stack. The purpose of the coating is to maximize the transmission at a single, multiple or a broad range of wavelengths. It is these products that would benefit from a surface structure as a replacement to the traditional coating.
The disadvantages of traditional coatings are: 1) the thermal expansion of the coating differs from the substrate, and during use, coatings can become separated from the substrate; 2) coatings have a shorter lifetime than that of the substrate material; and 3) coatings on plastic products are extremely expensive when compared to the price of plastic products. What is needed is an anti-reflective structure that is integrated into the substrate itself thereby having identical thermal expansion properties, equal lifetime and if integrated into a mold can be fabricated in the plastic product in a single step.
Extensive use is made of infrared (IR) imaging systems on helicopters, combat vehicles, missiles, and in man-portable equipment. Systems whose sensitivity and resolution exceed those of the currently fielded second generation devices are required for a new generation of devices which are “smart” enough to transfer part of the burden of target acquisition and identification from the soldier to the device itself to satisfy the requirements of Future Combat Systems. The infrared Focal Plane Array (FPA), which serves as the artificial retina for these systems, contains as many as about 106 pixels where optical detection and charge readout occur. In high performance FPAs, the ternary semiconductor alloy HgCdTe is used for optical detection and Si is used for the readout integrated circuit (ROIC). Arrays now in production are “hybrid” packages where detector and ROIC arrays are fabricated on separate manufacturing lines and subsequently indium-bump bonded to each other pixel by pixel to provide paths for charge flow and image transfer. In future arrays, the functions of detection and read-out are expected to be monolithically integrated on a single silicon chip, thereby eliminating the need for the indium-bump process.
A schematic cross sectional drawing of a portion of a HgCdTe detector array of the conventional art is given in FIG. 1a. Individual mesas 1 etched into the “front” surface of the HgCdTe epilayer 2 are the location of the individual photovoltaic pixels. Radiation 3 from a scene arrives at the back surface 9 of the CdznTe substrate wafer 4. A portion of the radiation crosses the back surface 9 and then the interface between the substrate 4 and epilayer 2. The shaded region underneath each mesa is the “active” area 5 where photons are absorbed and converted to electrons, which give rise to a current, which is proportional to the intensity of the absorbed radiation.
The efficiency of the conventional art device shown in FIG. 1a is depicted in FIG. 1b. The efficiency of the conventional art device depends on the amount of radiation 8 which reaches the active area 5. When radiation of wavelength λ encounters an interface 6 between two media (such as a vacuum and the back surface 9 of a FPA). A fraction R(λ), also called R, of the radiation 7 is reflected from the interface 6 and (1−R) 8 crosses the interface 6. To achieve maximum photo detection efficiency, it is of interest to minimize the value of R. However, a smooth vacuum to CdznTe interface has an abrupt change in refractive index from unity to 2.67, which limits the transmission value to 80% in the 8–12 μm spectral region. This problem is addressed on second generation FPAs by depositing a dielectric coating onto a surface of the FPA substrate 4.
Also, even in the dark, the active region in a photodetector is a source of noise currents whose magnitude is proportional to the volume of the active area 5. To minimize this effect, and thereby increasing the signal to noise ratio of a mesa diode, the volume of the active layer 5 should be minimized. A tradeoff clearly exists between the requirements of maximizing the signal by intercepting a large fraction of the incoming radiation and minimizing the source of noise by minimizing the volume of the active layer.
Antireflective coatings have fundamental and practical limitations. In principle, they can be designed to minimize R only at a single wavelength. When R is to be minimized over broad spectral bands such as the 3–5 μm and the 8–12 μm atmospheric windows utilized for target detection, a compromise must be made. Secondly, mismatches between thermal coefficients of expansion of coating layer and CdZnTe or Si substrate wafers often lead to peeling of the coating when FPAs are cooled from room temperature to a 77° K. operating temperature. An alternative to a dielectric coating, proposed more than a decade ago, is a periodic corrugated structure etched into the surface of an optical element such as a wafer. The effective change in the refractive index at the interface can be made vanishingly small by a proper choice of shape, period, and depth and the effective value of R thereby minimized. If the period is less than the wavelength of the incident radiation by at least a factor of two, then the structure is antireflective over a broad range of incident angles and wavelengths. For IR radiation, antireflective structures (ARS) would have periods on the order of several microns. However, such corrugated structures exhibit limitations due to the rectangular (binary) shape.
As has been shown, an array of photovoltaic diodes fabricated in HgCdTe epitaxial layers on CdZnTe substrates is the baseline detector architecture for high performance infrared focal plane arrays (IRFPAs). In contrast, current generation arrays have a disadvantageous thin dielectric antireflective layer deposited onto the substrate surface to enhance coupling of the incident radiation into the wafer. However, a smooth vacuum to CdZnTe interface has an abrupt change in refractive index from unity to 2.67 which limits the transmission value to 80% in the 8–12 μm spectral region. Modern FPA technology now requires transmission values above 80%. To achieve this, an interface with roughness on a scale smaller than the wavelength of the incident radiation would have an effective change in refractive index which is gradual, thereby consequently achieving an optical transmission value considerably higher than 80%.